Strain Gage Sensors “in” Concrete – The Hard Facts
Civil engineers constantly face the challenge of making accurate strain measurements in very hostile environments. Wind, rain sleet, snow, extreme temperatures, traffic...the list goes on and on.
Strain gages can, of course, be directly bonded to steel structures and even to the surface of concrete. The main challenges, when bonding to concrete are proper surface preparation and environmental protection. The porosity of concrete makes this a special challenge. See Micro-Measurements TechTip-611 for detailed instructions on product selection and procedures for bonding strain gages to concrete.
An even greater challenge comes when the test calls for embedded strain gages—that’s right, embedded! Protecting a delicate strain gage installation from the forces and impact of flowing concrete, the heat of the cure cycle and the tenacity of water in its attempt to permeate every nook and cranny is a daunting task for even the most experienced strain gage installation technician.
The common approach is to bond strain gages to ’sister bars’ which are tie-wired to the rebar prior to pouring the concrete. While this method works well from a strain measurement standpoint, getting the installation to survive is no small task, making the survivability of the installation uncertain with no chance of doing the job a second time.
Micro-Measurements has the solution. The EGP-Series Embedment Strain Gage is specially designed for measuring mechanical strains inside concrete structures. The sensing grid, constructed of a nickel-chromium alloy (similar to Karma), has an active gage length of 4 in [100 mm] for averaging strains in aggregate materials. A rugged 5 in [130 mm] outer body of proprietary polymer concrete resists mechanical damage during pouring, minimizes reinforcement of the structure, and provides protection from moisture and corrosive attack. The grid, cast within the polymer concrete to ensure maximum strain sensitivity, is self-temperature-compensated to minimize thermal output when installed in concrete structures. Each gage incorporates a heavy-duty 10 ft [3 m] cable with 22-AWG [0.643 mm dia.] lead wires; a three-wire construction to the sensing grid helps minimize temperature effects in the instrumentation leads. Special lengths of pre-attached cable will be quoted upon request. Micro-Measurements M-LINE accessory cable 322-DJV is available for adding cable length in the field.